A conventional four-cycle internal combustion engine has four strokes within a single combustion cycle: an intake stroke in which charge gas and fuel are introduced into each combustion cylinder, a compression stroke in which the charge gas and fuel mixture is compressed by the movement of a piston within the cylinder, an expansion stroke in which combustion of the mixture drives the piston and rotates a crankshaft, and an exhaust stroke in which exhaust generated by the combustion of the mixture is forced out of the cylinder by the movement of the piston. Combustion of the mixture is generally initiated by a spark produced by a spark plug associated with the cylinder, and the timing of the spark is critical to the performance of the engine. Spark timing is the process of setting the crankshaft angle at which the spark will fire relative to piston position and crankshaft angular velocity. Typically, for optimal performance, the spark is timed to fire near the end of the compression stroke at some point before the piston reaches the top dead center (“TDC”) position to generate maximum cylinder pressure for the expansion stroke. The spark timing must be advanced ahead of TDC because the combustion process and expansion of the resultant exhaust gas take a finite period of time. Accordingly, the crankshaft angular velocity (i.e., engine speed) affects the time frame in which the combustion process should occur to produce optimal power and/or fuel efficiency.
Many factors influence the desired ignition spark timing for a given engine, including the timing of the intake valves or fuel injectors, the type of fuel used, engine speed and load, air and engine temperature, intake air pressure, and other operating conditions. Newer engines typically use an engine control computer with a “spark map” (e.g., lookup table) with spark timing values for all combinations of engine speed and engine load. The control computer sends a signal to an ignition coil at the time indicated in the spark map to fire the spark plug and initiate combustion. However, conventional spark maps are generally based on steady-state conditions of the engine, such as engine speed and mass air flow into the engine. Consequently, changes in engine operating conditions, such as changes in the mass air flow, create periods of inefficient operation until a new steady-state condition is reached.
Under certain operating conditions, the appropriate spark timing for optimal power and/or fuel efficiency provided by the spark map can be counter-productive, producing an abnormal combustion process, including misfires and/or engine knock. “Knock” occurs when the mixture of fuel and charge gas burned in the engine generates such high combustion temperatures that one or more pockets of the mixture explode outside the envelope of the combustion flame front. Effects of engine knock range from inconsequential to completely destructive.
Knock may be attenuated by various methods, including among others retarding the spark timing (i.e., delaying spark timing until closer to TDC) and the use of exhaust gas recirculation. However, retarding the spark timing tends to decrease engine power and efficiency. Exhaust gas recirculation tends to lower combustion temperatures by replacing some of the oxygen (i.e., air) in the cylinder with mostly inert exhaust gas, which has a higher thermal capacitance than air and further suppresses combustion temperatures. Exhaust gas recirculation may also slow the combustion process, which affects the optimal spark timing. Further, exhaust gas recirculation generally does not respond quickly enough to prevent knock when the engine operating conditions change rapidly, for instance, when engine speed and mass air flow into the engine change in response to operator demand.
The issue of knock or misfire during transients is further more complex in internal combustion engines with dedicated exhaust gas recirculation. In dedicated exhaust gas recirculation architectures, the exhaust from one or more dedicated cylinders is routed directly to the inlet flow of the engine such that all the exhaust gas from these cylinders is pushed into the intake. Because generally no control valve is used in such a configuration, the amount of exhaust gas recirculation going back into the cylinders is unregulated and hence can vary more during transients. A conventional spark timing command based on a pre-determined table leads to an increased propensity for knock or misfire. Accordingly, there remains a need for further contributions in this area of technology to spark timing under such changing conditions.